In the United States, most industrial furnaces or boilers either use a stoker or a pulverized coal burner for burning the fuel. Stokers are less efficient than pulverized coal burners and are more limited in controlling the combustion process and the formation of gas pollutants. While pulverized coal burners are more efficient than stokers, they also have certain disadvantages. For example, pulverized coal burners require expensive pulverizing and handling equipment with high operating costs for mechanical power. Thus, pulverized coal units are only typically economically feasible for large installations. Stokers, on the other hand, are often preferred over pulverizers because of their greater operating range, their capability of burning a wide range of solid fuel and their lower power requirements.
The functions of a stoker are (1) to feed fuel to a furnace combustion zone at a steady but easily controllable rate, (2) to admit an evenly distributed supply of air to the fuel, (3) to retain the burning fuel in the combustion zone until complete combustion of all of the fuel, and (4) to separate the ash without permitting much air leakage. The efficiency of a stoker is largely measured by the completeness with which it combusts the fuel and how well this can be accomplished while maintaining a low percentage of excess air. Obviously, the efficiency of a stoker-type furnace depends largely on the type of fuel being burned.
There are several different types of stokers, such as an overfeed type, a single retort type and a traveling grate type. Examples of some prior art furnaces with traveling grate type stokers are disclosed in U.S. Pat. Nos. 697,620 to Green et al; 3,152,562 to Cohan et al; 4,510,873 to Shigaki; and 4,876,972 to Mrklas.
In view of the depletion of natural energy sources, the increased interest in combusting alternative waste materials and the increased concerns regarding air pollution, stoker type furnaces need to be more efficient and produce less pollutants. Accordingly, it is apparent that there exists a need for improving the efficiency of stoker type furnaces as well as a need to decrease the amount of pollutants produced by stoker type furnaces. This invention addresses these needs in the art, along with other needs which will become apparent to those skilled in the art once given this disclosure.